A gravity driven cooling system (GDCS) is an emergency source of low pressure reactor coolant used following a loss of coolant event in at least one known boiling water reactor (BWR). A typical GDCS includes pools of coolant positioned so that when coolant from the pools must be supplied to the reactor pressure vessel (RPV), the coolant flows, under gravity forces, through the GDCS coolant delivery system into the RPV. Under normal reactor operating conditions, however, coolant from the GDCS does not flow into the RPV.
To prevent such unwanted GDCS coolant flow during normal reactor operations, a squib valve typically is positioned in the coolant delivery system of the GDCS. The squib valve remains closed, and prevents GDCS flow, under normal reactor operations. When the RPV water level falls below some predetermined magnitude and the RPV is depressurized, the GDCS squib valve opens and coolant from the GDCS pool flows into the RPV.
When coolant flows from the GDCS pool to the RPV, it is desirable to prevent backflow of water from the RPV into the GDCS pool if the RPV pressure is higher than the GDCS driving force. To prevent such backflow, a GDCS biased-open check valve typically is positioned in the GDCS coolant delivery system. The GDCS biased-open check valve prevents reactor coolant backflow from the RPV to the GDCS pool once the GDCS squib valve has opened, or fired, initiating GDCS flow.
The GDCS biased-open check valve is an important component of the GDCS and must perform its function reliably when required. To ensure that the biased-open check valve performs its function when required, periodic surveillance tests are performed on the check valve. If the check valve fails the surveillance tests due to a problem with the valve, e.g., check valve hinge pins are frozen in place, the valve is repaired or replaced.
A known biased-open check valve used for GDCS applications includes a disk attached to a rotatable shaft supported by bearing surfaces in the valve body. The disk restricts flow through the valve body and is normally open about 15 degrees. The disk may open to a maximum angle of 60 degrees. To prevent backflow, the disk closes the flow channel through the valve body by seating on a valve body seat.
To ensure the operability of GDCS check valves, full flow testing of installed check valves under design conditions typically is performed. The GDCS, however, cannot be operated while the reactor is in operation, otherwise a loss of coolant accident will result. Therefore, the GDCS check valves are typically tested during refueling outages. Specifically, during a refueling outage, and for each GDCS check valve, a backflow is injected through the valve to rotate the disk by 15 degrees to the closed position. If a valve does not close, then the valve is repaired or replaced.
Such backflow test, however, cannot be performed during normal reactor operation. Therefore, if any GDCS check valve failures occur during reactor operation, such failures may not be detected until the next reactor outage. In addition, with the above described test, the valve is not tested over its entire range of motion.
It would be desirable to test the GDCS check valves while the reactor is in operation, rather than only during refueling outages, and not interfere with valve operation if the check valves are required to function. It also would be desirable to stroke the valve disk of each GDCS check valve over its entire range of motion to ensure that the valve has a full range of operability.